Polymerization shrinkage is one of the major deficiencies in dental polymers. Public and private research groups are expending considerable effort to develop materials and methods to reduce this shrinkage. The researchers require accurate, reproducible and pertinent measurements of shrinkage properties to assess the effectiveness of their developments. Researchers have developed several methods to assess the stress developed by dental filling composites as they shrink, but none of these previously developed methods have the ability to rapidly and accurately track stress in real-time under conditions approximating those of actual clinical use. Thus, there exists a great need in the art for an apparatus and method that overcome the deficiencies of prior methods and provide for measuring temporal stress development under curing and load conditions similar to those found in actual clinical use.
Such an apparatus and method is ideally capable of varying load and compliance settings to approximate the dynamic displacement occurring in teeth as dental fillings harden. Practitioners can use these settings to compare process parameters such as material composition, curing methods, surface area to volume ratio, and curing dynamics in real-time using samples that are similar in volume to dental fillings. Such an apparatus and method should provide for rapid loading and analysis of samples. Such an apparatus and method should also be automated to allow for rapid run condition and data collection, graphing and analysis. Such an apparatus and method will have utility for research and development of improved dental composites and any polymer where shrinkage and stress are important factors. Such an apparatus can also be used for research and development of improved initiator systems and curing devices. Such an apparatus and method could also be used in a manufacturing setting for polymer and composite quality assessment.
Many researchers have attempted to develop an apparatus and method for measuring polymerization shrinkage stress, but no researcher has yet developed an apparatus and method that is versatile enough to mimic clinical conditions and environmental conditions of clinical use. For example, Bowen provided the first reported description of a method for measuring shrinkage (Bowen R L, Adhesive bonding of various materials to hard tooth tissues. VI Forces developing in direct-filling materials during hardening. J Am Dent Assoc 1967 February;74(3):439-445). Bowen's method included placing samples between two platens of an Instron Universal Testing Machine. A load cell attached to the upper platen measured the load generated as the sample cured and shrank. A Tuckerman optical interferometer measured the displacement of the platens, and an operator manually adjusted the Instron crosshead to compensate for this displacement. Using this method, the practitioner calculated stress from the measured load and sample area and plotted the stress vs. time during the curing process.
The Bowen method is deficient, because the method involves measuring stress developing under near zero strain, since the practitioner compensates for the strain by manually adjusting the crosshead during the curing process. The condition maintained by the practitioner does not simulate the conditions that occur in clinical situations where teeth bend as the shrinkage stress increases. Bowen's method does not provide for mimicking the strain experienced in teeth. Bowen's method also does not provide for introducing other environmental factors to the test, such as light curing, water sorption, and convenient adjustment of bonded area/volume ratio (C-factor). Bowen's method also requires the tedious manual adjustment of crosshead position via manual movement of the crosshead drives.
Davidson improved upon Bowen's method (Davidson C L, deGee A J, Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984 February;63(2):146148) by adding an automated feedback transducer to perform the crosshead adjustment. Feilzer, in turn, added the ability to change the C-factor by adjusting the sample diameter and thickness and also added the ability to light cure the material (Feilzer A J, de Gee A J, Davidson C L, Setting stress in composite resin in relation to the configuration of the restoration. J Dent Res 1987 November;66(11)1636-1639). The method still, however, did not simulate the strain experienced in teeth during the curing of a filling. Additionally, specimens often fractured during testing due to the feedback requirement to maintain near zero specimen strain.
Many have adopted the Bowen/Davidson/Feilzer methodology, but none have resolved the problems of simulating tooth strain or of specimen fracture during testing. Also, none have adapted the methodology to provide for the addition of environmental factors, such as temperature change or water sorption.
Feilzer also introduced a method that involved measuring the curvature of a glass slide that was bent by the shrinkage stress of a composite sample bonded to one side of the slide (Feilzer A J, de Gee A J, Davidson C L, Relaxation of polymerization contraction shear stress by hygroscopic expansion. J Dent Res 1990 January;69(1)36-39). The method provided for determining stress by calculating the tangential bending stress of the slide to determine a maximum shear stress occurring at the ends of the sample strips. The experimental conditions, however, did not mimic in any way the strain conditions experienced in clinical settings where stresses are primarily wall-to-wall tensile stresses. The method also does not provide for adjusting the C-factor to be clinically relevant to bonded dental fillings.
Watts described a method similar to Feilzer's, involving a disc-shaped specimen cured between two glass plates (Watts D C, Cash A J, Determination of polymerization shrinkage kinetics in visible-light cured materials: methods development. Dent Mater 1991 October;7(4):281-287). The method included measuring the glass deflection to determine the kinetics of shrinkage volume change, but did not include stress measurements.
Watts described a second method of determining shrinkage stress using a cantilever beam shrinkage-stress kinetics in resin-composites: methods development, Dent Mater 20003 January;19(1):1-11). The cantilever beam deflection was measured with an attached strain gauge as the sample shrinkage pulled the beam downward. The sample was also attached to a load cell to record load generation during shrinkage. A correction factor was then applied to the raw stress values to normalize the data in an approximation to what were considered to be the expected stresses. The device was claimed to be useful for both light cured and chemically cured materials. One deficiency of the device was that it was designed with a fixed compliance and it did not have the ability to vary stiffness to simulate the different stress/strain characteristics of different tooth-restoration configurations. The device also required an estimated correction factor multiplier of 4 to arrive at the reported stress values derived from the beam deflection and load cell. The sample geometry could not simulate the bonded/unbonded area ratio found in tooth restorations and often used in these types of experiments and no provisions were made for monitoring the onset and completion of light curing during the measurement process. The device described did not provide for introducing environmental variables such as water sorption or temperature changes. No provisions were made for rapid sample loading and no direct calibration method was incorporated into the instrument design.
Researchers have also utilized finite element modeling to calculate stress development during shrinkage (Katon T R, Winkler M M, Stress analysis of a bulk-filled class V light.cured composite restoration. J Dent Res 1994 August;73(8)14701477). Finite element modeling methods can theoretically model two and three-dimensional filling configurations, but the stress values obtained are based upon engineering equations and assumptions of the basic mechanical properties of the materials and the substrates. Finite element modeling does not involve the testing of actual samples.
Researchers have also used photoelastics to determine stress locations and relative amount of stress in simulated fillings (Kinomoto Y, Torii M, Photoelastic analysis of polymerization contraction stresses in composite restorations. J Dent 1998 March;26(2):165-171). Again, no direct stress or load measurements are made using these methods, and interpretation relies upon assumptions of material and substrate properties. Also, the photoelastic models required do not have stress/strain relationships similar to real teeth.
Sakaguchi introduced a strain gauge method that combined finite element modeling with strain measured by embedded strain gauges in a sample (Sakaguchi R L, Ferracane J L, Stress transfer from polymerization shrinkage of a chemical-cured composite bonded to a pre-cast composite substrate. Dent Mater 1998;14(2):106-111). The method provided for tracking stress kinetics, but was not versatile enough to provide for varying the C-factor or include many of the clinically relevant factors, such as thermal expansion or water sorption. The method also did not provide for simulating the strain conditions reported in the literature for composite shrinkage in tooth cavities.
All of the methods and apparatus previously described in the literature fail to provide a quick, convenient, and clinically relevant method of determining shrinkage stress.